'\"
'\" Copyright 1989 Regents of the University of California
'\" Permission to use, copy, modify, and distribute this
'\" documentation for any purpose and without fee is hereby
'\" granted, provided that this notice appears in all copies.
'\" The University of California makes no representations about
'\" the suitability of this material for any purpose.  It is
'\" provided "as is" without express or implied warranty.
'\" 
'\" $Header: /cvsroot/PROCPLACE/pptinytcl/doc/Fork.3,v 1.1.1.1 2001/04/20 15:03:06 karl Exp $ SPRITE (Berkeley)
'\" 
.so man.macros
.HS Tcl_Fork tcl
.BS
.VS
.SH NAME
Tcl_Fork, Tcl_WaitPids, Tcl_DetachPids \- manage child processes
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
int
\fBTcl_Fork\fR( )
.sp
int
\fBTcl_WaitPids\fR(\fInumPids, pidPtr, statusPtr\fR)
.sp
int
\fBTcl_DetachPids\fR(\fInumPids, pidPtr\fR)
.SH ARGUMENTS
.AS int *statusPtr
.AP int numPids in
Number of process ids contained in the array pointed to by \fIpidPtr\fR.
.AP int *pidPtr in
Address of array containing \fInumPids\fR process ids.
.AP int *statusPtr out
Address of place to store status returned by exited/suspended process.
.BE

.SH DESCRIPTION
.PP
These procedures keep track of child processes in order to make it
easier for one application to manage several children.
If an application uses
the UNIX \fIfork\fR and \fIwait\fR kernel calls directly,
problems occur in situations like the following:
.IP [1]
One part of an application creates child C1.  It plans to
let the child run in background, then later wait for it to
complete.
.IP [2]
Some other part of the application creates another child C2,
not knowing anything about C1.
.IP [3]
The second part of the application uses \fIwait\fR to wait for C2
to complete.
.IP [4]
C1 completes before C2, so C1 is returned by the
\fIwait\fR kernel call.
.IP [5]
The second part of the application doesn't recognize C1, so it
ignores it and calls \fIwait\fR again.  This time C2
completes.
.IP [6]
The first part of the application eventually decides to wait
for its child to complete.  When it calls \fIwait\fR there are
no children left, so \fIwait\fR returns an error and the
application never gets to examine the exit status for C1.
.PP
The procedures \fBTcl_Fork\fR, \fBTcl_WaitPids\fR, and \fBTcl_DetachPids\fR
get around this problem by keeping a table of child processes and
their exit statuses.
They also provide a more flexible waiting
mechanism than the \fIwait\fR kernel call.
Tcl-based applications should never call \fIfork\fR and
\fIwait\fR directly;  they should use \fBTcl_Fork\fR,
\fBTcl_WaitPids\fR, and \fBTcl_DetachPids\fR.
.PP
\fBTcl_Fork\fR calls \fIfork\fR and returns the result of
the \fIfork\fR kernel call.
If the \fIfork\fR call was successful then \fBTcl_Fork\fR also
enters the new process into its internal table of child
proceses.
If \fIfork\fR returns an error then \fBTcl_Fork\fR returns that
same error.
.PP
\fBTcl_WaitPids\fR calls \fIwait\fR repeatedly until one of the processes
in the \fIpidPtr\fR array has exited or been killed or suspended by a
signal.
When this occurs, \fBTcl_WaitPids\fR returns the process
identifier for the process and stores its wait status at
\fI*statusPtr\fR.
If the process no longer exists (it exited or was killed by a signal),
then \fBTcl_WaitPids\fR removes its entry from the internal
process table.
If \fIwait\fR returns a process that isn't
in the \fIpidPtr\fR array, \fBTcl_WaitPids\fR saves its wait
status in the internal process table and calls \fIwait\fR again.
If one of the processes in the \fIpidPtr\fR array has already
exited (or suspended or been killed) when \fBTcl_WaitPids\fR
is called, that process and its wait status are returned
immediately without calling \fIwait\fR.
.PP
\fBTcl_WaitPids\fR provides two advantages.  First, it allows
processes to exit in any order, and saves their wait statuses.
Second, it allows waiting on a number of processes simultaneously,
returning when any of the processes is returned by \fIwait\fR.
.PP
\fBTcl_DetachPids\fR is used to indicate that the application
no longer cares about the processes given by the \fIpidPtr\fR
array and will never use \fBTcl_WaitPids\fR to wait for them.
This occurs, for example, if one or more children are to be
executed in background and the parent doesn't care whether
they complete successfully.
When \fBTcl_DetachPids\fR is called, the internal process
table entries for the processes are marked so that the
entries will be removed as soon as the processes exit or
are killed.
.PP
If none of the pids passed to \fBTcl_WaitPids\fR exists in
the internal process table, then -1 is returned and \fIerrno\fR
is set to ECHILD.
If a \fIwait\fR kernel call returns an error,
then \fBTcl_WaitPids\fR returns that same error.
If a \fIwait\fR kernel call returns a process that isn't in
the internal process table,  \fBTcl_WaitPids\fR panics and
aborts the application.
If this situation occurs, it means that a process has been
created without calling \fBTcl_Fork\fR and that its exit
status is about to be lost.
.PP
\fBTcl_WaitPids\fR defines wait statuses to have type \fIint\fR,
which is correct for POSIX and many variants of UNIX. 
Some BSD-based UNIX systems still use type \fIunion wait\fR for
wait statuses;  it should be safe to cast a pointer to a
\fIunion wait\fR structure to \fI(int *)\fR before passing
it to \fBTcl_WaitPids\fR as in the following code:
.nf
.RS

\fBunion wait status;
int pid1, pid2;
\&...
pid2 = Tcl_WaitPids(1, &pid1, (int *) &status);\fR
.RE
.fi

.SH KEYWORDS
background, child, detach, fork, process, status, wait
.VE
